1. Field of the Invention
The present invention relates to a GPS receiver apparatus which acquires specific information from a GPS signal and information acquisition method thereof.
2. Description of the Related Art
A GPS receiver apparatus which utilizes the global positioning system (GPS) receives radio waves sent from satellites and measures a current position of the apparatus. Currently, various types of GPS receiver apparatus are used depending on the intended purposes. The GPS receiver apparatus captures and tracks a plurality of satellites (four or more satellites in general) and acquires the whole navigation message sent from the satellites to calculate a position of the receiver apparatus at high speed and with high accuracy.
As shown in FIGS. 6A and 6B, the navigation message has a hierarchical structure that includes 25 main frames each of which includes 5 sub-frames. Each of the sub-frames includes 10 words. The main frames are sent every 30 seconds, the sub-frames are sent every 6 seconds, and the words are sent every 0.6 seconds. As a master frame, 25 main frames are sent every 12.5 minutes. The second word in each of the sub-frames contains data referred to as a hand over word (HOW), and the HOW includes accurate time information provided by the GPS satellites, i.e., time of week (TOW) that is a value expressed in units of 6 seconds beginning at 00:00:00 on every Sunday. Therefore, usage of the time information allows current clock time held by a clock circuit contained in the GPS receiver apparatus or any equipment including the GPS receiver apparatus to be accurately corrected.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2007-263598 discloses a technique to reduce power consumption required for time correction in a GPS receiver apparatus having a display function. According to this technique, when correcting a clock time once a day, the GPS receiver apparatus firstly starts supplying power to a tuner circuit and a signal processing circuit, the former of which receives radio waves from GPS satellites via an antenna, and the latter of which decodes reception signals contained in the received radio waves in parallel and detects a navigation message. When the time correction operation is completed, the power supply to the tuner circuit and the signal processing circuit is immediately terminated.
The above-described navigation message includes not only the time information in the sub-frames but also date information in the first sub-frame. That is, the third word in the first sub-frame contains data referred to as a week number (WN) (see FIG. 7). Described in the WN word is the number of weeks past from the week starting from 00:00:00, Jan. 6, 1980, where a week number of this week is set to “0”. Therefore, when the GPS receiver apparatus has a calendar function, the current date can be automatically set (or corrected) using the date information, as required by a user at the time of firstly starting usage of the apparatus or at the time of initializing the apparatus.
The HOW is contained in all sub-frames; however, the WN word is contained only in the first sub-frame as described above. Thus, when receiving the navigation message so as to acquire the date information, it is desirable to fortunately receive the first sub-frame immediately after a satellite is captured. However, when the second sub-frame is received immediately after the satellite is captured, for example, reception of the radio waves from the satellite should be continued until the first sub-frame can be received next time. Therefore, acquisition of the date information to set the current date consumes significantly large power in comparison with acquisition of the time information. Especially when the GPS receiver apparatus uses a coin cell battery or other types of small battery (including rechargeable battery), continuous operation time of the apparatus is largely reduced.